Natural Weapons Against an Ancient Plague

By WARREN E. LEARY

Published: January 4, 1994

WASHINGTON—
LOCUSTS and grasshoppers, the stuff of Biblical plagues, still ravage the earth, stripping vegetation in their path and depriving millions of people of food. But after decades of battling these insects with toxic chemicals, scientists are developing a new generation of natural pesticides that are expected to be less hazardous to the environment and more selective in reaching their targets.

Instead of spraying chemicals on marauding clouds of locusts, scientists are experimentally introducing fungi and other natural enemies to carry on the fight.

Like most other insects, locusts and grasshoppers are susceptible to infection by viruses, bacteria, fungi and other organisms. Hoping to use this natural vulnerability, researchers have been looking at infectious agents that destroy locusts and grasshoppers, which are close cousins, without affecting people, other animals, plants and benign insects.

Agriculture Department scientists, for example, discovered a protozoa called Nosema locustae that selectively kills grasshoppers but not other insects. The single-celled parasite destroys grasshoppers by eating their fat. And the scientists have tested a wasp imported from Australia that lays its eggs on grasshopper egg pods, producing larvae that eat developing hoppers.

"Everyone wants an alternative to chemical pesticides because we know the harm that they do to the environment," said Dr. John E. Henry, a retired Agriculture Department entomologist who has conducted research on alternatives for combating locusts and grasshoppers for three decades. During the last major locust outbreak in Africa, which reached a peak in 1988, Western industrial nations refused to provide the most effective insecticide, dieldrin, because they were concerned that the potent poison, which can stay in the soil for years and is suspected of causing cancer, would enter plants and animals, ending up in milk and food consumed by humans.

"Biological alternatives are the way to go," said Dr. Henry, an adjunct professor at Montana State University. "They are the future."

But there are skeptics. "Similar things have been tried in the past and, under optimal conditions, could be effective," said Dr. David Pinentel, a professor of entomology at Cornell University. "But you don't often have perfect conditions in the real world, so I'll be somewhat skeptical until I see positive results."

In addition, some scientists wonder if the biological pest killers are as specific and benign as the proponents believe. Even if an agent targets only grasshoppers, critics ask, what is the effect on grasshopper species that are not crop pests?

Locusts remain a major agricultural problem in the developing world, and grasshoppers are a persistent pest in the United States, mainly in cattle-grazing areas of the West where they damage grasslands. Experts estimate that grasshoppers cost the nation $400 million to $500 million a year in lost crops and grass, and they say the damage rises in major outbreaks, which occur every 8 to 10 years. In the third world, locusts or grasshoppers can destroy enough food to leave millions of people hungry and they strip away so much vegetation that soil becomes susceptible to erosion.

Locust outbreaks frequently occur when rains end a drought, providing ideal moist conditions for locust reproduction. The infestation in northern and sub-Saharan Africa, the worst in more than 30 years, began in 1986 with the end of a drought.

The United Nations Food and Agriculture Organization, headquartered in Rome, worked with the Desert Locust Control Organization of East Africa and other groups to attack the locusts in the earliest stages of the outbreak. This effort was thwarted, however, partly because the infestation started in areas rife with wars and civil strife, including the Sudan, Chad and the western Sahara, where locust hunters could not operate, officials said.

With this foothold and ideal weather conditions, locusts swept from the semiarid areas of east Africa to Senegal on the west coast, and some swarms rode strong winds across the Atlantic Ocean to reach the Caribbean for the first time in contemporary history, experts said. One swarm sighted in Algeria covered 150 square miles and contained an estimated 20 billion insects, each capable of eating more than its weight in food each day.

Locusts and grasshoppers, with their large hind legs for jumping, look much alike, although locusts have shorter antennae and many are slightly larger. There are an estimated 5,000 species of locusts and grasshoppers, including more than 300 in the contiguous United States. Most Promising New Pesticides

The insect cousins differ primarily in behavior. Grasshoppers are solitary insects that seldom band together, experts said, although at times of overpopulation they can form large clusters and ravage vast stretches of land. Locusts also have a solitary phase, but spend much of their time in swarms of millions or billions, traveling large distances and eating everything green in their path. History has recorded locust swarms so large that they blackened the sky and covered hundreds of square miles.